The present application relates to the art of medical diagnostic imaging in which penetrating radiation is received by an array of radiation sensitive detectors. The invention finds particular application in conjunction with fourth generation computerized tomographic (CT) scanners and will be described with particular reference thereto. However, it is to be appreciated that the invention may also find application in conjunction with other diagnostic imaging modalities.
Heretofore, CT scanners have included a plurality of discrete radiation detectors arranged in a ring or a rotatable ring segment around a patient examination region. Each detector included a radiation sensitive face, such as a scintillation crystal which converted received radiation into a corresponding quantity of light. A solid state photodiode or vacuum tube photomultiplier converted the light emitted by the scintillation crystal into electrical signals indicative of the intensity of emitted light, hence the intensity of received radiation.
The size of the radiation sensitive face was typically one of the most significant parameters determining spatial resolution of the CT scanner. That is, the width of the path or ray of radiation as defined by the x-ray source and the detector was highly dependent upon the width of the detector. Detail within the object or patient being scanned which had physical dimensions approximately equal to or less than the x-ray path width was difficult, if not impossible, to distinguish. By making the width of the radiation detector narrower, the width of the radiation path or ray was narrowed. Narrowing the radiation path decreased the width of the region of the patient which contributed to the resultant electrical signal, hence, improved spatial resolution. Of course, decreasing the width of the radiation sensitive surface also decreased the number of photons of radiation received per unit time, hence, detection efficiency.
In this manner, there is a trade off between radiation dose and resolution when deciding whether to make the detectors wide or narrow. Depending on the nature of the study or diagnostic image desired, either wide or narrow detectors may be advantageous. In order to select between the advantages of wide and narrow detectors, prior scanners have selectively placed an aperture plate or septa in front of the detectors to restrict the radiation impinging thereon. The radiation plates could be removed when the benefits of a wide detector were desired and inserted when the higher resolution benefits of a narrow detector were desired. Moreover, aperture plates of the varying sizes could be configured for mechanical insertion in front of the radiation receiving face of the detectors, enabling any one of a plurality of detector widths to be utilized.
One disadvantage of the use of aperture plates is that the plates or septa that define the aperture block radiation which has traversed the patient from impacting the radiation sensitive face of the detector. Thus, this radiation to which the patient has been exposed does not contribute to the resultant image, reducing dose utilization and radiation detection efficiency. Another approach was to collimate the radiation beam adjacent the radiation source to define a plurality of narrow beams which impinge upon the patient and detectors. This type of collimation was impractical with CT scanners that utilize a large number of closely spaced detectors and/or where relative motion existed between x-ray source and detectors.
Another approach which attempted to combine the dose utilization advantages of a wide detector with the resolution of a narrow detector is illustrated in U.S. Pat. No. 4,398,092 to R. Carlson, et al. This patent discloses a relatively wide detector which is more sensitive to radiation impacting the center of the detector than to radiation impacting the edges. However, this non-uniform weighting of x-ray photons, which results from the non-uniform response of the detector, increases the quantum noise, i.e. the amplitude of the photon statistics, since minimal quantum noise is achieved when all photons are weighted equally. Moreover, in Carlson only those photons near the center of the detector contain useful higher spatial frequency information and hence, the response to higher frequencies is greatly reduced compared to the low frequency response. In fact, at certain spatial frequencies, the non-uniform detector response can actually cause a further decrease in the high frequency response, a condition that can result from a phase reversal in the signal of the edge detected photons. Due to the low response to high frequencies, this system is of limited benefit in scanning patients.
Another technique for reducing the width of the necessary detectors was to place the detector ring of a fourth generation scanner inside of the rotational path of the x-ray tube. This configuration reduced the diameter or circumference of the detector ring, hence the width of the individual detectors. However, this geometry was disadvantageous in that it required a rather complex motion, called nutating, of the detector ring to allow the x-ray beam to pass. This geometry was also susceptible to scatter radiation due to the close proximity of the detectors to the patient.
Moreover, it should be noted that each of these prior art systems use a ring of radiation detectors in which all radiation detectors had the same width in the peripheral direction.